1. Field of the Invention
This invention relates to a method for manufacturing compound semiconductor epitaxial wafer, more particularly to a method for manufacturing compound semiconductor epitaxial wafer having an alloy composition gradient layer.
2. Description of the Prior Art
In a field of manufacturing red light emitting diode or those emit orange or yellow lights, generally used is a compound semiconductor epitaxial wafer composing a single-crystalline substrate made of gallium phosphide GaP or gallium arsenide GaAs on which an alloy composition constant layer made of gallium arsenide phosphide GaAs.sub.1-a P.sub.a (where, "a" represents a real number satisfying 0.ltoreq.a.ltoreq.1) in which GaAs and GaP, the III-V compound semiconductors not being comprised in the single-crystalline substrate, conform with predetermined alloy compositions of "1-a" and "a", respectively. Emission wavelength is determined by the alloy composition "a", which is typified by a=0.9 for yellow light emitting diode, a=0.65 for orange light and a=0.57 for red light, when a gallium phosphide GaP single-crystalline substrate is used.
When a relatively large lattice mismatch is produced between the compound semiconductor single-crystalline substrate made of gallium phosphide GaP or gallium arsenide GaAs and the GaAs.sub.1-a P.sub.a alloy composition constant layer grown thereon, misfit dislocation will occur at the interface to relax the stress due to the lattice mismatch.
The dislocation, however, will be causative of degrading luminous efficiency of a light emitting diode if it extends to the alloy composition constant layer where an emission region is formed.
General solution for preventing the misfit dislocation from extending is to interpose, between the single-crystalline substrate and the GaAs.sub.1-a P.sub.a alloy composition constant layer, a GaAs.sub.1-x P.sub.x alloy composition gradient layer in which an alloy composition "1-x" of gallium arsenide GaAs and an alloy composition "x" of gallium phosphide GaP vary in a gradual manner. As a method for forming such alloy composition gradient layer, it is known to make gradual changes in a composition of a source gas fed into an environment for growing the alloy composition gradient layer, as well as making changes in vapor phase growth temperature (Japanese Patent Laid Open Showa 49-11468).
Here, the change in the vapor phase growth temperature is intended for improving a crystallinity of the GaAs.sub.1-x P.sub.x alloy composition gradient layer. In a process of generating the compositional variation of the alloy composition gradient layer from that of the single-crystalline substrate toward that of the GaAs.sub.1-a P.sub.a alloy composition constant layer accompanying with epitaxial growth, that is, to make a GaAsP layer epitaxially grown on a GaP substrate while increasing a compositional ratio of As in the source gas, the vapor phase growth temperature will be gradually lowered when the alloy composition "1-x" of gallium arsenide GaAs is raised. On the contrary, the vapor phase growth temperature will be gradually elevated when the alloy composition "x" of gallium phosphide GaP is raised, that is, to make a GaAsP layer epitaxially grown on a GaAs substrate while increasing a compositional ratio of P in the source gas.
After a series of research and development for blocking extension of the misfit dislocation toward the GaAs.sub.1-a P.sub.a alloy composition constant layer and for effectively relaxing the stress due to lattice mismatch within the epitaxial layer, the present inventors have previously found out that it was preferable to make the alloy composition be sharply increased during epitaxial growth of the alloy composition gradient layer and, immediately after that, to make the alloy composition be partially recovered at a moderate A rate (Japanese Patent Application Heisei 8-22029), which is totally opposed to the conventional method producing gradual changes in alloy composition as has been believed in common.
It was, however, made clear that an attempt to make sharp changes in vapor phase growth temperature corresponding to variation in the alloy composition, while making sharp changes also in the composition of a source gas containing group V elements in the periodic table (simply referred as group V source gas hereinafter) to be fed, to obtain the alloy composition gradient layer internally having sharp changes in the alloy composition, would often result in failure because of a large thermal capacity of a susceptor on which a substrate is placed, and a limited heat transmission rate from the susceptor to the substrate.
Instantaneous excessive heating and cooling to achieve drastic changes in the vapor phase growth temperature will result in slip dislocation more easily to degrade the crystal quality.